1. Field of the Invention
The present invention relates to stepper motors, and more particularly, to such a motor which is comprised of planar elements.
2. Description of the Prior Art
A stepper motor is an electromechanical device that converts electrical energy, delivered in the forms of pulses, into mechanical energy which is represented as discrete rotational or linear movements. Such a motor consists of two major elements--a stator which can include a coil having a number of windings and a rotor which can include permanent magnets. Electromagnetic interaction between these two elements causes the rotor to make a step movement for each polarity change in the stator windings. There is a direct correspondence in stepper motors between the energization of a particular stator coil and the position of the rotor. Thus, they are ideal open loop devices and are widely used in a variety of positioning applications such as printer paper feeds and small clocks. Optical encoders or magnetic Hall effect sensors can be used in stepper motors to close the control loop in order to obtain the maximum torque or acceleration from a given stepping motor or to improve its positioning accuracy.
In one known type of stepper motor, shown in U.S. Pat. No. 4,517,478, the rotor includes a permanent magnet in the form of a thin disc with magnetized alternate poles evenly distributed around its periphery. The motor also includes two stator halves, each of which contains four coils and a plurality of circumferentially-spaced pole pieces of high magnetic permeability. When the stator halves are assembled together, the pole pieces become aligned with each other to form C-shaped poles which form a small air gap in which the disc magnet of the rotor rotates. When current pulses are applied to the stator coils, the resulting magnetic flux penetrates the disc magnet, creating a combination of attracting and repelling forces between the stator and magnet poles that result in a step-by-step rotation of the rotor. A disadvantage of this motor is that the stator arrangement is very complex and is difficult and expensive to manufacture.
In a second type of stepper motor, as shown for example in U.S. Pat. No. 4,501,981, the motor comprises two axially-spaced helically wound stator coils, and a rotor mounted for rotation within the coils. The rotor includes a cylindrical multipolar permanent magnet which is radially magnetized. Each of the stator coils is provided with a field ring which includes regularly-spaced axial teeth forming salient pole pieces for the respective stator armatures. The use of the cylindrical permanent magnet rotor and the field rings with axial teeth make the motor relatively large and heavy, and thus, the motor is not suitable for applications, such as a drive motor for an autofocus mechanism, where a light compact motor is required.